1. Field of the Invention
The present invention relates to a magneto-resistance effect element and a thin-film magnetic head.
2. Description of the Related Art
Magnetic disk drives employ a thin-film magnetic head having a magneto-resistance effect element (MR element) for reading magnetic signals. In recent years, efforts have been made to design magnetic disk drives for higher recording densities, and accordingly there are growing demands for thin-film magnetic heads, particularly magneto-resistance effect elements, which satisfy higher-sensitivity and higher-output requirements.
A CIP-GMR (Current in Plane-Giant Magneto-resistance) element which is a giant magneto-resistance effect element having a nonmagnetic layer between ferromagnetic layers and passing a sensing current in parallel to a layer surface, has been conventionally developed as a reproducing element in a thin-film magnetic head. On the other hand, a magnetic head that uses a TMR (Tunnel Magneto-resistance) element which has an insulation layer instead of the nonmagnetic layer as an intermediate layer and which passes a sensing current perpendicular to a layer surface, has also been developed in order to achieve higher densification. Furthermore, a magnetic head that uses a CPP (Current Perpendicular to Plane)-GMR element which is a GMR element having a nonmagnetic layer as the intermediate layer and passing a sensing current perpendicular to the layer surface similar to the TMR element, has also been developed. CPP-GMR element has an advantage of having low resistance in comparison with the TMR element and having higher output in a narrower track width than the CIP-GMR element.
An ordinary GMR element is in the cylindrical shape of a desired size, and has a structure interposing a non-magnetic spacer layer between a pinned layer which is a ferromagnetic layer in which the magnetization direction is fixed a and a free layer which is a ferromagnetic layer in which the magnetization direction varies according to an external magnetic field. Such a GMR element is also referred to as a spin valve film (SV film). The upper and lower ends of the GMR element are provided with a cap layer and a buffer layer, respectively. The cap layer, the GMR element, and the buffer layer are interposed between the upper shield layer and the lower shield layer. In the case of the CPP-GMR element, the upper shield layer and the lower shield layer function as an electrode, respectively, and a sense current flows in a direction orthogonal to the layer surface.
Heretofore, most of the layers that make up a CPP-GMR element are made of a CoFe alloy or a NiFe alloy. If such a CPP-GMR element has a practical read gap of about 40 nm, then the MR ratio of the CPP-GMR element is of a low value of about 4%.
Japanese Patent Laid-Open No. 2003-2418428 discloses that a CPP-GMR element having a high MR ratio of 10% or higher is obtained if a free layer and a pinned layer are made of a Heusler alloy. Specifically, it is known that the higher the MR ratio the-greater is the spin polarizability of the free and pinned layers, and the MR ratio increases if the free and pinned layers are made of a material having a high spin polarizability. It is also known that the material for realizing a half metal which is a magnetic material having spin polarizability of 100% or nearly 100% is a Heusler alloy. If Heusler alloy layers are thus used in the free and pinned layers, then the MR ratio increases for an increased output.
Japanese Patent Laid-Open No. 2006-5282 discloses a CPP-GMR having a pinned layer which includes a plurality of magnetic layers, some of which are made of a Heusler alloy.
As disclosed in the above patent documents, if the free and pinned layers of a CPP-GMR element contain a Heusler alloy typified by CoMnSi and CoMnGe, then it is possible to improve MR ratio.
However, if a Heusler alloy is to exhibit a high spin polarizability, then the Heusler alloy has generally to be annealed at a high temperature. For example, CoMnSi which is a typical Heusler alloy, exhibits a high spin polarizability when it has been annealed at a temperature of 300° C. or higher. However, it is not preferable for the process for fabricating CPP-GMR elements to include steps at a temperature of 300° C. or higher, and all the steps of the process of fabricating CPP-GMR elements should be carried out at a temperature of 290° C. or lower. This is because, if an MR element is subjected to a temperature of 300° C. or higher, Permalloy (NiFe) of the shield layer undergoes grain growth, resulting in a reduction of magnetic permeability.
On the other hand, another typical Heusler alloy, CoMnGe, exhibits a high spin polarizability even when it has been annealed at temperatures ranging from 270° C. to 290° C. Therefore, CoMnGe is practically preferable over CoMnSi as a Heusler alloy for use in the free and pinned layers of a CPP-GMR element.
However, CoMnGe has a very large magnetostriction in a regularized state. While the magnetostriction of 90Co10Fe and 80Ni20Fe as measured by the optical lever method is represented by about ±3×10−6, the magnetostriction of regularized CoMnGe as measured by the optical lever method is represented by about ±1×10−5.
The pinned layer whose magnetization direction is fixed by an exchange coupling with the antiferromagnetc layer poses no problem even if it has a certain high level of magnetostriction. However, if the free layer whose magnetization direction is variable, depending on the external magnetic field, has a high level of magnetostriction, then the waveform symmetry is undesirably lowered. Therefore, a high level of magnetostriction of the free layer is not preferable.
The waveform symmetry refers to an accurate proportionality relationship from the point of origin, between the external magnetic field due to the recording medium or the like and the output voltage of the MR element, as shown in FIG. 1. Specifically, the waveform symmetry represents, when the external magnetic field is 0 A/m, the output voltage is 0 mV, when a positive magnetic field is applied, a positive output voltage is produced, and when a negative magnetic field is applied, a negative output voltage is produced. Usually, the waveform symmetry is expressed as a percentage of asymmetry. It is empirically known that asymmetry of an MR element is optimum if it is 0% (as indicated by the solid-line curve in FIG. 1) and is allowable in a range of ±10% (as indicated by the broken-line curves in FIG. 1). However, if the asymmetry of an MR element is large (as indicated by the two-dot-and-dash-line curves in FIG. 1), then a thin-film magnetic head incorporating the MR element fails to provide a desired reading performance. The relationship between the magnetostriction of the free layer and the waveform symmetry as shown in FIG. 2 indicates that the waveform symmetry is in an allowable range if the magnetostriction of the free layer is represented by about 6.5×10−6 or lower. Therefore, it is necessary that the magnetostriction of the free layer be kept at the level of 6.5×10−6 or lower. As described above, regularized CoMnGe fails to satisfy the requirement, though 90Co10Fe and 80Ni20Fe satisfies the requirement. It has thus been practically difficult to employ a CoMnGe film in the free layer.